27-4 Objectives

1. 27-4 Objectives • Identify the basic characteristics that make the outer planets different from terrestrial planets. • Compare the characteristics of the outer planets. • Explain why Pluto is now considered a dwarf planet.

2. The Outer Planets • gas gianta planet that has a deep massive atmosphere, such as Jupiter, Saturn, Uranus, and Neptune. • The four planets furthest from the sun, Jupiter, Saturn, Uranus, and Neptune, are called the outer planets and are separated from the inner planets by a ring of debris called the asteroid belt. • Usually found past the orbit of Neptune is Pluto. At one time, Pluto was considered the most distant planet. But in 2006, Pluto was reclassified as a dwarf planet. • A dwarf planet is a celestial body that orbits the sun, is round because of its own gravity, but has not cleared its orbital path.

3. Gas Giants • Gas giants are larger and more massive than terrestrial planets, but much less dense. • Each planet probably has a core made of rock and metals. • Each gas giant has a thick atmosphere made mostly of hydrogen and helium gases. • Unlike terrestrial planets, gas giants did not lose their original gases during formation. • All four gas giants have ring systems that are made of dust and icy debris that orbit the planets.

4. Jupiter • Jupiter is the fifth planet from the sun. • Jupiter is the largest planet in the solar system and has a mass more than 300 times that of Earth. • The orbital period of Jupiter is almost 12 years. Jupiter rotates on its axis faster than any other planet—once every 9 h and 50 min. • Jupiter has at least 60 moons. • It also has several thin rings that are made up of millions of particles.

5. Jupiter, continued Jupiter’s Atmosphere • Hydrogen and helium make up 92% of Jupiter, so Jupiter’s composition is much like the sun. • Jupiter never became a star, like the sun, because it did not have enough mass to allow nuclear fusion to begin. • The orange, gray, blue, and white bands on Jupiter’s surface suggest the presence of organic molecules mixed with ammonia, methane, and water vapor. • Jupiter also has lightning storms and thunderstorms that are much larger than those on Earth.

6. Jupiter, continued Weather and Storms on Jupiter • Jupiter’s Great Red Spot is an ongoing, massive, hurricane-like storm that is about twice the diameter of Earth. • Several other storms can be seen, although they are usually white. The Galileo spacecraft measured wind speeds up to 540 km/h on Jupiter. Jupiter’s Interior • Jupiter’s large mass causes the temperature and pressure in Jupiter’s interior to be much greater than they are inside Earth. • With temperatures as high as 30,000ºC, Jupiter’s interior is a sea of liquid, metallic hydrogen. Scientists think that Jupiter has a solid, rocky, iron core at its center.

7. Saturn • Saturn is the sixth planet from the sun and is the least dense planet in the solar system. • The orbital period of Saturn is 29.5 years. Saturn rotates on its axis every 10 h and 30 min. • Saturn is very cold and has an average cloud-top temperature of –176°C. • Saturn has at least 47 moons. • Like Jupiter, Saturn is made almost entirely of hydrogen and helium, and has a rocky, iron core at its center.

8. Saturn, continued Saturn’s Bands and Rings • Saturn is known for its rings, which are 2 times the planet’s diameter, and is a complex and extensive system of rings. • Saturn’s rings are made of billions of dust and ice particles, probably from comets or other bodies. • Like Jupiter, Saturn has bands of colored clouds that run parallel to its equator. • Saturn’s rapid rotation, paired with its low density, causes Saturn to bulge at its equator and to flatten at its poles.

9. Uranus • Uranus is the seventh planet from the sun and the third-largest planet in the solar system. • The orbital period of Uranus is almost 84 years. • Uranus has 24 moons and at least 11 thin rings. • Discovered by Sir William Herschel in 1781, Uranus is a difficult planet to study because it is nearly 3 billion kilometers from the sun. • The Hubble Space Telescope has taken images to show changes in Uranus’s atmosphere.

10. Uranus, continued Uranus’s Rotation • Most planets rotate with their axes perpendicular to their orbital planes as they revolve around the sun. However, Uranus’s axis is almost parallel to the plane of its orbit. Uranus’s Atmosphere • Unlike the other gas giants, Uranus has an atmosphere that contains mainly hydrogen and helium. • The blue-green color of Uranus indicates that the atmosphere also contains significant amounts of methane. • Scientists think has a core of rock and melted elements with a temperature of about 7,000ºC.

11. Neptune • Neptune is the eighth planet from the sun. • The orbital period of Neptune is nearly 164 years. Neptune rotates about every 16 h. • Neptune is similar to Uranus in size and mass. • Neptune has at least 13 moons and six rings.

12. Neptune, continued The Discovery of Neptune • Neptune’s existence was predicted before Neptune was actually discovered. Scientists suspected that the gravity of unknown planet was responsible for the variations in Uranus’s orbit. Neptune’s Atmosphere • Neptune’s atmosphere is made up mostly of hydrogen, helium, and methane. • Neptune has the solar system’s strongest winds, which exceed 1,000 km/h. • The Great Dark Spot on Neptune was a giant storm the size of Earth that appeared and disappeared on Neptune’s surface.

13. Objects Beyond Neptune • Pluto, now defined as a dwarf planet, orbits the sun in an unusually elongated and tilted ellipse. • It spends most of its orbital period beyond Neptune’s orbit, but is sometimes closer to the sun than Neptune. • Scientists think Pluto is made up of frozen methane, rock, and ice, with an average temperature of –235°C. • Pluto has extensive methane icecaps and a very thin nitrogen atmosphere.

14. Kuiper Belt Objects • Kuiper Belt a region of the solar system that is just beyond the orbit of Neptune and that contains dwarf planets and other small bodies made mostly of ice • In recent years, scientists have discovered hundreds of objects in our solar system beyond Neptune’s orbit. • Some of these objects are similar to Pluto in size and composition. • At least one of these object, named Eris, is also a dwarf planet. Eris is slightly larger than Pluto. • Sedna, one of the most distant objects in the solar system, was found beyond the Kuiper belt, is three-fourths the size of Pluto.

15. Exoplanets • Exoplanets are planetlike bodies that orbit stars other than Earth’s sun. • Exoplanets cannot be directly observed with telescopes or satellites. • Most exoplanets can be detected only because their gravity tugs on stars that they orbit. • Most of the exoplanets that have been identified are larger than Uranus, but a few have been discovered that are closer to Earth in mass.